Conventionally known and market-released semiconductor memory devices such as DRAM, SRAM, and flash memory have all used a MOSFET as a memory cell. Therefore, along with miniaturization of patterns, it has been required to improve dimensional accuracy at a rate exceeding the rate of miniaturization. As a result, a large burden has also been placed on lithography technology for forming these patterns, which has been a major cause of a rise in product costs.
In recent years, resistance varying memory has been receiving attention as a successor candidate of such semiconductor memory devices employing a MOSFET as a memory cell. Such a resistance varying memory has advantages that a cross-point type cell structure in which memory cells are formed at intersections of intersecting bit lines and word lines can be adopted, making miniaturization easy compared to conventional memory cells, and also a stacking structure can be configured in a longitudinal direction, making it easy to improve a degree of integration of a memory cell array.
A write operation (setting operation) of data to a resistance varying memory of so-called bipolar type is performed by applying to a variable resistance element a setting voltage of a first polarity. This causes the variable resistance element to change from a high-resistance state to a low-resistance state. On the other hand, an erase operation (resetting operation) of data is performed by applying to a variable resistance element in a low-resistance state after a setting operation a resetting voltage of a second polarity which is opposite to the first polarity applied during the setting operation. This causes the variable resistance element to change from a low-resistance state to a high-resistance state.
When performing this kind of setting operation or a forming operation (an operation to render a memory cell in a state of being transition-capable between a high-resistance state and a low-resistance state), it is important to suppress unwanted sneak currents. This is because if sneak currents are large, a required value of a forming voltage increases, whereby problems arise such as trouble occurring in the forming operation, power consumption increasing, and so on.